Forced-flow boiler of the oncethrough type



March 27, 1962 R. MICHEL 3,026,857

FORCED-FLOW BOILER OF THE ONCETHROUGH TYPE Filed March 19, 1958 15 TUBES 3O TUBES m 30 TUBES 3,026,857 FORCED-FLOW BOILER OF THE ONCE- THROUGH TYPE Rupprecht Michel, Erlangen, Germany, assignor to Siemens-Schuckertwerlre Aktiengesellschaft, Berlin-Siemensstatlt, Germany, a corporation of Germany Filed Mar. 19, 1958, Ser. No. 722,587 Claims priority, application Germany Mar. 30, 1957 3 Claims. (Cl. 122-235) My invention relates to forced-flow steam generators of the once-through type wherein the feed water is heated, evaporated and superheated on a single passage through the tube system of a boiler.

When water is being heated in such a boiler and, along its travel path, is gradually converted to steam, the specific volume of the working medium in the tubes also increases in accordance with the increase in temperature. The flow cross section along the path of the working medium must be adapted to such volumetric increase. it is customary, therefore, to increase the tube cross section in steps or to branch the tubes at one or several locations into a larger number of parallel tubes of the same inner diameter thus providing for the desired increase in total cross section. a

It has been proposed to place the branching point at a location where the evaporation of the working medium has already commenced. It has further been proposed to locate the branching point an additional distance away from the point of commencing evaporation, namely into a zone in which about 30 to about 50% of the water is already evaporated in normal operation.

In a more particular aspect, my invention specifically relates to such forced-flow boiler tube systems of the branched type, and 'has for its object to minimize or reduce the tendency toward instable operation observed with such tube systems and thus to improve the degree of uniformity of the steam-generator output and/or to reduce its pressure and temperature-regulating requirements.

According to my invention, the first portion of the parallel-tube system, located flow-wise ahead of the abovementioned branching point, is designed in the shape of meander bands that extend upwardly and downwardly in a vertical or nearly vertical direction; and the next following evaporator portion of the tube system, located flow-wise behind the interposed collector or branching means and comprising a greater number of parallel tubes than the first system portion, is designed in the shape of meander bands that extend substantially horizontally and, as a whole, pass upwardly away from the branching point.

When firing the boiler with pulverized coal, it is preferable to let the boiler tubes extend as much as possible in a vertical direction at the height of the burners in order to avoid the danger of ash deposition or slagging on the tubes. However, it is readily possible, according to the invention, to cover at least one of the four combustionchamber walls with horizontally extending meander-arranged tubes. In boilers with fire boxes of the meltingchamber type, also called slag-tap or slag-pool chambers, the melting chamber proper, as far as it is provided with retaining pins and with stamped mass, can readily be lined with horizontal tubes. It is further possible, for the purpose of the invention, to mount horizontal tubes at locations behind the ash catcher grate or ash-dumping grate.

The above-mentioned tendency toward instability of operation in forced-flow steam generators with paralleltube systems stems mainly from the necessity of arranging the tubes vertically upward and downward at the height of the burners and is caused by the difference in States Patent 3,026,857 Patented Mar. 27, 1962 height at respective points of the upwardly and downwardly extending parallel tubes. This tendency is particularly disagreeable at low load and when starting the steam-generating operation. Such difliculties are minimized or eliminated by virtue of my invention due to the fact that only the first portion of the parallel-tube system, located at a place where such arrangement is absolutely necessary, has its tubes extend upwardly and downwardly, whereas the same tube system, after being branched into a higher number of parallel tubes, has its subsequent parallel tubes arranged in form of horizontal meander bands along a stepwise ascending path. That is, since these evaporator tubes contain at their entrance a large proportion, such as about 50% by weight, of steam, they do not tend to produce instable operation although they are arranged horizontally, provided the meander shape of the tube group follows a continuous though incremental upgrade.

The above-mentioned object, advantages and features of my invention will be more fully explained with reference to the drawing, in which:

FIG. 1 shows schematically a forced-flow steam generator according to the invention;

FIG. 2 relates to the same embodiment and illustrates schematically the disposition of the tube system in the firing chamber of the boiler; and

FIG. 3 illustrates schematically a modified form of a forced-flow steam generator according to the invention.

The tube system illustrated in FIG. 1 has its heating surfaces composed of vertical and horizontal groups of tubes, the relative position of these heating surfaces being apparent from FIG. 2. The feed Water enters in the direction of the arrow 1 into a collector 2 and thence flows into fifteen parallel tubes of the heating surface 3, the tubes extending in meander shape upwardly and downwardly. After leaving the heating surface 3 in which the feed water is pre-heated but still liquid, the water passes through an equal number of tubes along an unheated connecting portion 4 and thence serially into the heating surface 5, another connecting portion 6, and another heating surface 7'. From heating surface 7, the

water passes through a connecting portion 8 into a branching collector 9 from which the water is distributed onto twice the number of horizontal tubes which extend upwardly in steps and which together form another meander-shaped heating surface 10 of the tube system. As mentioned, much of the water is evaporated at the entrance of heating surface 10, and the residual liquid is fully converted to steam either in heating surface 10 or in serially subsequent heating surfaces.

In FIG. 2, showing the forced-flow boiler in section, the cylinder-shaped firing chamber, denoted by A, is of the cyclone type and forms the primary firing chamber. Chamber A is provided with an opening B for igniting the cyclone and for observing the firing operation. Primary air and fuel are blown spirally into the cyclone A and are ignited. The ignited mixture then passes through an opening C of the heating surface 3 into the secondary combustion chamber D of the boiler. Chamber D is separated from the tertiary chamber E by the heatingsurface wall 7. Since the tubes in the upper portion of the heating surface 7 are firmly welded together, the combustion gases are forced to flow downwardly through the lower portion of the heating surface 7, which also serves as an ash catcher grate, into the chamber E where they impart their residual heat to the evaporator-heating surface It -In the embodiment illustrated in FIG. 3, the tube systern is provided with parallel groups of tubes of the same diameter. Located on two mutually opposite side walls of the boiler and also on a third side of the fi-ring chamber are groups of parallel tubes 11, 12, 13 which extend in meander shape upwardly and downwardly. Each of these groups is formed, for instance, by fifteen tubes of 38 mm. inner diameter. After the working medium passes serially through the three groups 11, 12 and 13, it enters into a collector 14 from which the tube system is branched into thirty horizontal parallel tubes all of the same diameter and also arranged in meander fashion. The thirty parallel tubes form together a heating surf-ace 15.

It will be apparent to those skilled in the art, upon a study of this disclosure, that my invention permits of various modifications and hence may be embodied in tube systems and boilers other than those particularly illustrated and described herein, without departing from the essence of my invention and within the scope of the claims annexed hereto.

I claim:

1. In a forced-flow steam generator having a firing chamber and respective pre-heating and radiant heating and contact heating evaporating zones and having a system of boiler tubes extending through said zones, said tube system comprising a first portion of parallel tubes, a second portion of a greater number of parallel tubes than said first portion so as to provide in said second portion a larger total flow cross section than in said first portion, and branching means located within said radiant heating evaporating zone and serially interconnecting said two portions and located at a position where the proportion of steam in said system has reached approximately 50% by weight, said first portion being located flow-wise ahead of said branching means and having its parallel tubes arranged in a meander-shaped group extending upwardly and downwardly in substantially vertical direction within said firing chamber, said second portion behind said branching means having its larger number of parallel tubes arranged in a meander-shaped group extending substantially horizontally to form a continuously upward and horizontal flow path, and said second portion being connected at its bottom with said branching means to extend in meander steps upwardly therefrom.

2. In a forced-flow steam generator having a firing chamber with four side walls forming respective preheater and radiant heating and contact heating evaporator zones, and having a system of boiler tubes extending through said zones, said tube system comprising a first portion of parallel tubes, a second portion of a greater number of parallel tubes than said first portion so as to provide in said second portion a larger total flow cross section than in said first portion, and branching means serially interconnecting said two portions and located within said radiant heating evaporating zone at a location where the proportion of steam has reached approximately by weight, said first portion being located flow-wise ahead of said branching means and having its parallel tubes arranged in a meander-shaped group extending upwardly and downwardly in substantially vertical direction within said firing chamber, said second portion behind said branching means having its larger number of parallel tubes arranged in a meander-shaped group extending substantially horizontally to form a continuously upward and horizontal flow path, said second portion being connected at its bottom with said branching means to extend in meander steps upwardly therefrom and being disposed on one of said four fire-chamber walls.

3. In a forced-flow steam generator having a firing chamber with four side walls forming respective preheater and evaporator zones, and having an ash catcher grate in said chamber, the combination of a system of boiler tubes extending through said zones, said tube system comprising a first portion of parallel tubes, a second portion of a greater number of parallel tubes than said first portion so as to provide in said second portion a larger total flow cross section than in said first portion, branching means located within said evaporator zone at a location where the proportion of steam is approximately 50% by weight and serially interconnecting said two portions, said first portion being located flow-wise ahead of said branching means and having its parallel tubes arranged in a meander-shaped group extending upwardly and downwardly in substantially vertical direction within said firing chamber, said second portion behind said branching means having its larger number of parallel tubes arranged in a meander-shaped group extending substantially horizontally to form a continuously upward and horizontal flow path, said second portion being connected at its bottom with said branching means to extend in meander steps upwardly therefrom, and said second portion being disposed predominantly at a location of said chamber behind said grate relative to the flow of combustion gases.

References Cited in the file of this patent FOREIGN PATENTS 1,111,182 France Oct. 26, 1955 654,156 Germany Dec. 11, 1937 750,484 Great Britain June 13, 1956 770,456 Great Britain Mar. 20, 1957 

